Indwelling needle devices are widely used for treatment such as infusion, blood transfusion, and extracorporeal blood circulation. Leaving a metal needle inside a blood vessel may injure the blood vessel, and for this reason, Patent Document 1 discloses an indwelling needle device that includes a soft outer needle and a hard inner needle and is configured so that it is inserted into a blood vessel of the patient in a state in which the leading end of the inner needle protrudes from the leading end of the outer needle and then the inner needle can be retracted from the outer needle.
FIG. 8A is a cross-sectional view showing a leading end of an inner needle 920 protruding from a leading end of an outer needle 910 of the indwelling needle device disclosed in Patent Document 1 and the vicinity thereof. A side hole 921 is formed in an outer circumferential face of the inner needle 920. In the state shown in FIG. 8A, when the inner needle 920 and the outer needle 910 are sequentially inserted into a blood vessel of the patient, blood 930 of the patient flows inside the inner needle 920, and also passes through the side hole 921 of the inner needle 920 and flows inside a gap 925 between the inner needle 920 and the outer needle 910. When the outer needle 910 is composed of a transparent or translucent material, the blood flowing through the gap 925 can be visually observed, and flashback of the blood can be checked easily. Moreover, in addition to an inner cavity of the inner needle 920, the gap 925 can also be filled with the blood of the patient, so that in, for example, hemodialysis, a priming operation is facilitated, and the risk that blood in which air is mixed may be returned to the patient can be reduced.